1. Field of the Invention
The present invention relates to the determination of a quantization step size in a system which digitally codes picture information for transmission and storage of data. More particularly, the present invention relates to a coding method which adaptively varies a quantization step size so as to quantize a picture signal during intraframe coding or interframe coding, thereby stably maintaining the variation of data quantity stored in a transmission buffer, even when a scene change occurs. An apparatus employing an adaptively set quantization step is also disclosed.
2. Discussion of Related Art
In digital-transmitting or digital-processing a video signal, various coding methods have been used for the sake of reduction in transmission costs. Variable bit rate coding is a typical method for high efficiency coding. In variable bit rate coding, pulse code modulation is performed for a large amount of data during intraframe coding, while differential pulse code modulation is executed to produce the minimum amount of data during interframe coding. Video data, which has been pulse code modulated or differential pulse code modulated, is supplied to a transmission buffer via a quantizer or variable length coder, etc.
While the data quantity supplied to the transmission buffer is not constant, a transmission channel of digital data generally has a constant transmission rate. Accordingly, a coded signal is transmitted to the receiver at a constant transmission rate after first being stored in the transmission buffer. However, since the buffer does not have an infinite storage capacity for data, a quantization step size should be varied in accordance with the occupancy rate, by continuously checking the occupancy rate of data provided to the transmission buffer. This variation of a quantization step size prevents an overflow or underflow of the buffer.
A conventional coding method employs intraframe coding when a scene change occurs. The quantization step size is radically enlarged because the data occupancy rate in the buffer is raised due to an increase in the quantity of data. When a scene changes, a low occupancy in the buffer makes the quantization step size small, while a high occupancy in the buffer makes the quantization step size larger than that prior to changing the scene. Therefore, during a scene change there is a drawback that there is a remarkable difference in picture quality within one frame. Since the frames next to a scene changed frame, retain the previous high quantization step size due to the earlier high occupancy rate in the buffer, it takes a long time to improve picture quality.
European Patent Publication No. 0,444,839 A2 published on Sep. 4, 1991, discloses a system for resetting a quantization step size during a scene change. This reference discloses a technology which selectively intraframe codes and interframe codes a picture signal. In an exemplary case, a macro block of coded picture data is intraframe coded by a refresh, employing the quantization step size of the previous frame. On the other hand, when intraframe coding without a refresh is performed, it uses the largest quantization step size previously determined, thereby quantizing picture data and supplying the quantized data to a transmission buffer memory. Accordingly, the coding system disclosed by this reference prevents storage of an excessively large amount of quantized data in the buffer, thereby maintaining smooth picture transitions.